Created an enums.proto that contains the device idle modes,
which is referenced by batterystats.
Note that, currently, batterystats is the origin of these
constants in the java code, and I have kept that as is.
Thus, batterystats references the enum.
Nevertheless, because the actual control of device idle
mode is DeviceIdleController.java, where these constants
will likely one day be moved, I have put the constants
in server/enums.proto (since DeviceIdleController is
in server, not os like BatteryStats).
Bug: 69478930
Test: cts-tradefed run cts-dev -m CtsStatsdHostTestCases -t android.cts.statsd.HostAtomTests#testDeviceIdleModeStateChangedAtom
Change-Id: I6e66801ae8256aab423067f9a4b852a194564a8d
8bdae8d682